Reword intro to linter section in README

Now that `scss-lint` is more customizable, the wording of this section
was misleadingly suggestive that `scss-lint` was still opinionated.

Change the wording to say that the defaults are opinionated.

Change-Id: I28ea02b17361f5fc7bb3375862fc7eb69bf565a3
Reviewed-on: https://gerrit.causes.com/33622
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>